Manufacture of nitrided steel articles



' Nov. 9, 1943. E, R. GADD ET A. 2,334,081

MANUFACTURE OF NITRIDED STEEL ARTICLES Ffiled Nov. 14, 1939 wk 1g 1 19 ErrwsZE. GadcZ lzarrj 153' aas PatentedNov. 9', 1943 V 2.334.081. MANUFACTURE" OF NITRIDED STEEL ARTICLES Ernest Reg inald Gadd and Harry Keates, Bristol, England, assignors to The Bristol Aeroplane Company Limited, Bristol, England, a. British c pa y Application November 14, 1939, Serial No. 304,418 I In Great Britain November 1 2, 1938 v scl ims. (crux-4 This invention relates to the manufacture of nitrided steel articles of the 'kind which are.

made of austenitic steel and case-hardened by a nitriding process. the nitrided case the article must be finished practicallyto size before nitriding, and this is" done by machining and grinding. These operations have the efiect of work-hardening the surface material, destroying its austenitic structure and replacing it by another structure, such as martensite, which'is not responsive to nitriding.

Further, the machining and grinding 'operations and exposure to the atmosphere cause an oxide film to form-on the work-hardened surface, still further increasing its nitriding. i

In order to enable the nitriding process to be carried out it is therefore necessary to remove the resistant oxide film and .Iork-hardened sur-,

face. According. to the present invention, a method of treating the article prior to nitriding consists in immersing it in an electrolytic bath,

resistance to Owing to the-hardness of r the sleeve is assembled in the engine, transmits the driving movement from the driving mechanism to the sleeve. The bolts I! provide a convenient electrical connection between the conductor I9 and the sleeve, thesleeve being elec-" tically insulated from the rod l5 by the insulating bracket l6. At the sides of the sleeve rectangular lead plates 2|) are also supported in the container l4, being connected by conductors 2| to 'a-bus-bar 22, to which the rod I5 is connected in like manner.

The container it is filled with an electrolyte comprising potassium bichromate in dilute sul-.

phuric acid. The bath is prepared by adding from,42' to 56 parts by volume of commercial concentrated sulphuric acid to su'flicient, water etching it anodically to replace the resistant oxide film and work-hardened surface by a passive oxide film and then etching it cathodically to remove the passive film. I

It is found that the passive film produced by the anodic treatment renders the article susceptible to the nitriding process but that the presence of this film makes necessary such a high temperature for the nitriding process that pitting and distortion of the articlesare produced. The eflect of the cathodic treatment, by removing the passive film, is to enable a lower temperature to be used for the nltriding process. v

A specific embodiment of the invention will now be described as applied to a processfor the manufacture of sleeve-valves for controlling the ports of a sleeve-valve internal-combustion en gine. In the accompanying drawing:

Figure 1 is a sectional elevation of the electrolytic bath according to theinvention, and

Figure 2 is aplan view. After the sleeve in ,has been machined and ground it is suspended as shown in Figure l. with its axis vertical, by means of books it which engage the ports 12 in the sleeve and are fastened to a cross-bar [3 of insulating material which rests on the edges of a liquid container It.

A rod-shaped electrode l5 of metallic lead is supported with its axis coinciding with the axis of the 'sleeve as shown. The rod is supported to make up 100 parts by volume of dilute acid.

Owing to, the fact that the concentrated acid is hydroscopic and contains water, the concentration of the dilute acid will be from to by volume of pure acid, the lowest concentration being preferred. From 1 to 3 parts by weight of potassium bichromate is added to sufficient of the dilute acid to make up 100 parts by weight of the bath.

Anodic etching of the sleeve is efiected by connecting the conductor Hi to the positive pole of a source of directcurrent. As shown in Figure 2 the positive and negative supply conductors 23 and 26 respectively are connected either to the bus-bar 22 and conductor l9 respectively or vice versa by adjustment of a suitable changeover switch 25. For anodic etching the switch 25 is adjusted to the position shown in full lines so that the sleeve is connected to the positive pole and the lead rod l5 and lead plates 20 are connected to the negative pole. Current is passed for from four to eight minutes, six minutes being the preferred time, thecurrent density being between 0.75 and 0.85 ampere per square inch of sleeve. Thus, for a sleeve of 13" length and 5%" internal diameter, the currentused is between 350 and 400 amperes. 'However, the range of current density may vary from 0.5 to 1.0 ampere per square inch which, with a sleeve of the dimensions given above, means a by a bracket l8 of insulating material which is current of from 250 to 500 amperes.

stances for a shorter-period of time, not less than one minute.- The eflect of the cathodic etching, as above explained, is that thepasslve oxide film is removed.

The etching process is followed by nickel plating of those parts which are not to be nitrided. In some circumstances the parts which are to be nitrided are copper-plated. The sleeve is now placed, together with other etched sleeves, in suitable containers which are moved into a furnace where they are subjected to the action of an atmosphere of ammonia at 550 C. for '72 hours. If the sleeves are not subjected to the cathodic etching above described, the temperature of the nitriding treatment has to be raised to 620 C. to

' produce the same case. The lower temperature has the advantages that it reduces the distortion of the sleeves, reduces the amount of pitting of the nitrided layer and lengthens the life of the containers.

The advantage of the etching process according to the invention is that it is possible in some circumstances to dispense with copper-plating of the surfaces prior to nitriding; in particular, where the process is applied to the treatment of poppet-valves and other articles other than sleeve-valves.

Instead of a bath containing potassium bichromate, a bath containing glycerol may be used, the glycerol being added in the proportion of ,50 of an ounce avoirdupois of glycerol per gallon of dilute acid.

We claim:

1. The process of surface-hardening an austenitic steel article whereof the surface has been work-hardened by machining, which process comprises the steps of etching the article anodically in an electrolytic bath to replace the work-hardened surface. by a passive oxide film,

in the bath for a period of four to eight minutes at a current density of from 0.5 to 1.0 amperes per square inch of etched surface to replace the work-hardened layer by a passive oxide film, reversing the direction of the current for a period of from one to three minutes to etch the article cathodically and remove the passive oxide film, removing the article from the bath and subjecting the article to a nitriding treatment.

3. The process of surface-hardening an austenitic steel article whereof the surface has been work-hardened by machining, which process comprises the steps of immersing the article in an electrolytic bath comprising potassium bichromate in an aqueous solution of sulphuric acid in the proportion of from 1 to 3% by weight of potassium bichromate, etching the article anodically in the bath to replace the work-hardened surface by a passive oxide film, reversing the direction of they current to etch the article cathodically and remove the passive oxide film, removing the article from the bath. and subjecting the article to a nitriding treatment.

4. The process according to claim 3 wherein the aqueous solution'of sulphuric acid has a concentration in the order of parts by volume of pure sulphuric acid to 70 parts by volume of then etching the article cathodically to remove the passive film and finally subjecting the article to a nitriding treatment.

2. The process of surface-hardening an austenitic steel article whereof the surface has been work-hardened by machining, which process comprises the steps of immersing the article in an electrolytic bath, etching the article anodically water.

5. The process of surface-hardening an' austenitic steel article whereof the surface has been work-hardened by machining, which, process comprises the steps of immersing the article in an electrolytic bath comprising glycerol in an aqueous solution of sulphuric acid in the proportion of about of an ounce of glycerol per gallon of the solution, etching the article anodically in the bath to replace the work-hardened surface \by a passive oxide film, reversing the direction of the current to etch the article cathodicaliy and remove the passive oxide film, removing themticle from the bath and subjecting the article to a nitriding treatment.

ERNEST REGINALD GADD HARRY KEATES. 

